Lewis Hamilton completed a clean sweep of fastest times in Korean Grand Prix Friday practice as Mercedes again took the fight to Red Bull.
In another strong hint that the champion team will face tougher opposition this weekend than of late, Hamilton not only set the outright pace with a 1m38.673s, but was also very positive about tyre life over later long runs.
Sebastian Vettel was 0.108 seconds down on Hamilton in second place.
The runaway world championship leader, chasing a fourth consecutive Formula 1 race win this weekend, waited much longer than anyone else to move to super-soft tyres, jumping to second when he did so.
That denied Mercedes a one-two, with Vettel edging Nico Rosberg by 0.016s.
Mark Webber had been Hamilton's main rival early on, as the pair swapped first place several times. The second Red Bull ended up back in fourth, but was within 0.063s of its sister car.
Felipe Massa was Ferrari's pacesetter in fifth, two places ahead of team-mate Fernando Alonso. Romain Grosjean's Lotus was in between them.
The second Lotus of Kimi Raikkonen got straight back out after its morning crash and went eighth.
Despite a cockpit niggle, Jenson Button was ninth for McLaren, ahead of very-closely matched Force India duo Adrian Sutil and Paul di Resta, who were just 0.001s apart.
No one repeated Raikkonen's trip into the wall, but both Caterham's Charles Pic and Sauber's Esteban Gutierrez had spins during the afternoon while Alonso visited the grass on his final lap.
Pos Driver Team Time Gap Laps 1. Lewis Hamilton Mercedes 1m38.673s 31 2. Sebastian Vettel Red Bull-Renault 1m38.781s +0.108s 30 3. Nico Rosberg Mercedes 1m38.797s +0.124s 32 4. Mark Webber Red Bull-Renault 1m38.844s +0.171s 35 5. Felipe Massa Ferrari 1m39.114s +0.441s 30 6. Romain Grosjean Lotus-Renault 1m39.226s +0.553s 34 7. Fernando Alonso Ferrari 1m39.444s +0.771s 31 8. Kimi Raikkonen Lotus-Renault 1m39.757s +1.084s 33 9. Jenson Button McLaren-Mercedes 1m39.774s +1.101s 29 10. Adrian Sutil Force India-Mercedes 1m40.006s +1.333s 34 11. Paul di Resta Force India-Mercedes 1m40.007s +1.334s 34 12. Sergio Perez McLaren-Mercedes 1m40.152s +1.479s 31 13. Esteban Gutierrez Sauber-Ferrari 1m40.186s +1.513s 33 14. Nico Hulkenberg Sauber-Ferrari 1m40.210s +1.537s 34 15. Jean-Eric Vergne Toro Rosso-Ferrari 1m40.446s +1.773s 30 16. Daniel Ricciardo Toro Rosso-Ferrari 1m40.552s +1.879s 26 17. Pastor Maldonado Williams-Renault 1m41.117s +2.444s 35 18. Valtteri Bottas Williams-Renault 1m41.289s +2.616s 34 19. Giedo van der Garde Caterham-Renault 1m42.461s +3.788s 36 20. Charles Pic Caterham-Renault 1m42.798s +4.125s 35 21. Jules Bianchi Marussia-Cosworth 1m43.108s +4.435s 31 22. Max Chilton Marussia-Cosworth 1m43.441s +4.768s 29